Recovery of isoprene by fractionation and extractive distillation



Sel-L 2, 1947. J. A. PATTERsoN Er AL 2,426,705

RECOVERY 0F ISOPRENE BY -FRACTIONATION AND EXTRACTIVE DISTILLATION 2Sheets-Sheet 1 Filed Dec. 29. 1942 ll' AN.

NI. @NPN Sept. 2, 1947. J. A. PATTERSQN Er AL 2,426,705

RECOVERY OF AISOPRENE BY FRACTIONATION AND EXTR'ACTIVE DISTILLATION 2sheets-Sheet 2 Filed Dec. 29, 1942 portance.

Patented Sept. 2, 194,7`

RECOVERY OF ISOPRENE BY FRACTIOA- TION EXTRACTIVE DISTILLATION John A.Patterson, Westfield, and Harold W. Scheelne, Elizabeth, N. J.,assignors to Standard Gil Development Company, a corporation of Delawarev Application December 29, 1942,' Serial No. 470,426 y 1 l Thisinvention relates to the treatment `of mixtures ofhydrocarbons of abovethe C: range to effect the separation and purification of parafiins,mono-olefins, and diolens. The invention relates particularly to theseparation of diolefins containing carbon atoms in the molecule fromvtheir admixture with other hydrocaribons in products ofthe treatment ofpetroleum fractions; and also to the purification of diolefins.

An object of this invention is to provide an g improved method forconcentrating isoprene from a mixture of hydrocarbons of 5 carbon atomscontaining also pentenes, including 2- methyl butene-Z and cyclopentene,such as that obtained by cracking a petroleum oil, in which thehydrocarbon mixture is first distilled to pro- 2 Claims. (Cl. 202-39.5)

duce a distillate fraction substantially free of cyclopentene butcontaining 2-..methyl butene-2 and other pentenes, and this distillateis then subjected to extractive distillation in the presence of avolatility modifying solvent to effect separation of the oentenes as adistillate fraction from isoprene. This isoprene is then separated fromther solvent, which is yrecycled to the upper part of the extractivedistillation zone.

In the drawing, Figures 1 and 1A, taken together, constitute adiagrammatic showing of one embodiment of our process. 'i

From the various refining processes for the preparation of motor fuels,highly volatile liquid by-products are obtained. These liquid byproductsare today of great commercial im- Many of these liquid by-products aremixtures of low molecular-Weight hydrocarbons, and as such are difficultto resolveiinto useful mixtures by the ordinary processes of fractionaldistillation. Thus, for hydrocarbons containing 5 carbon atoms in themolecule the boiling points at 760mm. are:

. C. 3-methy1 butene-l 21.2 Pentene-l 30.2 12A-methyl butene-l 31.1Isopre'ne 34.1 Trans pentene-Z 35.9 n-pentane 36.0 Cis pentene2 l 37.02-methy1butene-2 38.4 Cyclopentadiene 41,0 Trans piperylene 41.9 Cispiperylene 43.9 Cyclopentene 44.1 Cyclopentane 49.5

, 2 It is to be noted that the tion of an individual constituent.Furthermore,

if a fractional distillation is madeL upon suchY cracking-coilby-products at extremely high reflux ratios to obtain very narrowfractions', pro` f longed heating of the mixture is involved; and,

as a result, considerable polymerization of the more unsaturatedcompounds commonly occurs. Thus, fractional distillation alone as ameans for separating very narrow fractions from such mixtures isdisadvantageous.

It has now been found that such complex mixtures may be advantageouslyresolved into their individual constituents by a unique combination ofprocessing steps. This invention involves at least three such processingsteps. In two of the three steps fractional distillation is employed.Between the two fractional distillation steps is effected an extractivedistillation. The process of the invention is thus the combination 'of acarefully controlled fractional distillation of a suitable feed stock,followed by an extractive distillation of'a particular distillatefraction, and then subsequently fractionally distilling one of theparticular distillate fractions from the extractive distillationoperation. The process of the invention therefore resides not only inthe combination of fractional distillation steps with extractivedistillation, but in the particular manner of combination in order toachieve particularly desirable fractions for .the second and the thirdsteps in the processing-namely, for the extractive distillation and thesecond fractional distillation steps. It is usual, also, in commervboilingv points of the various types of compounds inl the tabulation doj not follow the degree of chemical saturationr 'ofi the compounds. Thisis particularly noteworthy f since highly complex mixtures vof theCthydro-y` carbons are commonly producedy in large vol;7 umes asby-products from refinery lcracking unitsz" Extremely expensivefractionation equipmenty would be needed in order to effect the"segregal- 3 tionof -constituentshis based upon diier'ences at diiferenttemperatures "of lthe vapor pressures of selected groups of compounds inthe presence of 1 certain type liquids added to a fractional distillathevarious type hydrocarbons in such a way that in a narrow boilingfraction the parains will be the most volatile constituent; themono-oleflns,

liquids in an extractive less volatile; and the diolefins, the leastvolatile.

It is such lan effect of an added liquid under suitable conditions ofpressure and of concentration of theadded liquid that contributes to theformation of azeotropic compositions. In extractive distillation,however, as distinct from azeotropic distillation-the added liquid isemployed in substantially greater amounts in order to exert the maximumeffect upon certain groups of com pounds. f The amount of added liquidemployed in an extractive distillation is greatly in excess of thatwhich would form with themixture as a whole azeotropic mixtures of theconstituents thereof. Thus, the addition of the extraneous liquid inextractive distillationis related to-butv very distinct from-theaddition of a denite amount of the added liquid in anazeotropic dis-`tillation system. Y Y v Liquidssuitable in the extractive distillationhigh dielectric constant. In other words, suitable materials are thenormally liquid polar organic the following characteristics:

1. High solubility for the hydrocarbons; V2. Marked'eifect on relativevolatility;v 3.

distillation or when azeotropes are formed by solvent extraction, orother means; Suiiicient volatility to permit stripping without requiringthe use of high temperatures; Chemical stability when heated in thepresence of hydrocarbons; `6., Commercial availability andnon-corrosiveness.

vIn the case of the separation of the Cs hydro- Y carbons from theiradmixture with other hydrocarbons, acetone, furfural, pyridine, theamines,

foundtobe particularly advantageous. i

Taking the separation of isoprene as a suitable example of the processof the invention, the first` step involves the careful fractionation inmultiple-plate distillation equipment of a narrow Cs hydrocarbonfractiony containing a substantialy possible of the piperylene,cyclopentadiene,v and pentenes. A suitable source for isoprene is therefinery `bi1-product distillate from the high' temperature cracking ofgas oil, virgin naphtha, kerocal debutanized stock from this source maycontain about 4% isoprene.

A distillate of this nature is flrst carefully fractionated in amulti-plate distillation tower so as to derive therefrom as overheadmaterial a close boiling Cs fraction containing essentially parafstepare in general the stable organic liquids of compounds containingoxygen, nitrogen, sulfur,` and related elements. Suitable liquids alsohave and their admixtures with water, have been,v

sene, and/or other suitable feed stocks. A typi-y Readily separable fromthe hydrocarbons by after the separation of the added liquid is thenamount of isoprene; and, `rejecting as much as head product; and toleave as distillation residue clopentadiene-commonly occurs'duringtheex- 4. ns, mono-olefins, and diolei'lns; and, as a distillationresidue, hydrocarbons of higher boiling points of C5 to Csjmolecularcontent. In the rst fractional distillation step, separation is thusmade of a distillate containing as high a content of isoprene as isconsistent with the feed stock from the distillation residue containinga substantial quantity of the higher boiling `Cs hydrocarbons inassociation with hydrocarbons of 05+ molecular content. It is to berecognized that cyclopentadiene forms azeotropes with the pentanes andtherefore in the separation of the C5 hydrocarbons, if cyclopentadieneis not separated in the first fractionating tower, separation fromisoprene until the inal puriiication step may thus be difiicult.

The close boiling overhead fraction is then carefully distilled in thepresence of an excess of an organic liquid which affects in differentde-' grees the relative' volatilities of the variou's type hydrocarbons.For this purpose acetone, furfural, the liquid basic nitrogen compoundssuchv p as pyridine and the aminesand mixtures thereof with water aresuitable. In the case of acetone the amount of aqueous acetone employedis usu-e ally about twice the volume of the liquid hydrocarbon mixtureflowing down through the tower;

` while in the case of furfural the'ratio is often as high as 7 to 1.

Thus, under suitably controlled conditions in thek presence of suchquantities of added liquid, such as aqueous acetone, a vapor overheadproduct is obtained which consists essentially of parafns andmono-oleflns of C5 molecular content.

`'lhe distillation residue Which is normally obtained consistsessentially of solvent which is recycled to the proper point in thetower to maintain as high a solvent to hydrocarbon ratio as isadvantageous for the process. The most important fraction removed fromthe system is the intermediate distillate material conveniently termedthe side-stream product. This side-stream product consists mainly ofisoprene with such quantities of piperylene and cyclopentadiene as werepresent in the feed and such traces of liquid polymer products as wereformed during the distilla- I tion operations. Boththe overhead andsidestream distillate fractions may contain substan-- tial quantities ofthe added liquid-that is,l aqueous acetone in this caseas a result oftheformation of azeotroplc mixtures. The side-stream product from theextractive distillation operation is normally'treated to effect theseparation of the liquid added in the extractive -distillationoperation. When aqueous acetone is employed as the added liquid, theVsidestream product is usually countercurrently treated with water toremove the acetone, and thus to permit the isoprene and otherhydrocarbons to be separated therefrom. The hydrocarbon materialfractionally distilled under vcarefully controlled conditions to obtaina-s an overhead product one oi' the hydrocarbons in substantial purity.In the present case the fractional distillation is eiected to obtainsubstantially pure isoprene, as the overpiperylenes, the cyclopentadienenot previously removed due to the presence inthe system ofv pentanes,and smaller quantities of other mpurities.Y

Since some polymerization-particularly 0.5 cy.

tractve distillation'step, high-boiling polymeric materials are normally.equipment I through line II..

' plates.

. pressure uponv the the speciilc composition given is maintained aboutvpresent in the solvent recycle. In order to maintain the polymer contentof the solvent at a low value, usually about of the recycle solventstock is removed `and com- I bined with the overhead dstillateobtainedfrom the extractive distillation; and the combined stream is thentreated for solvent recovery.

In order to illustrate the invention more clearly.

the following detailed description of processing is presented. Thereference numerals given in the description referto the accompanyingldrawings which present a suitable lay-out of equipment, and indicates asuitable flow `of materials for processing according to the invention:In this illustration, example will be taken ofthe separation and.purication of isoprene as obtained from the-treatment of a debutanizeddistillate stream from' a liquid phase cracking of a virgin gas oilstock. A specic example of a typical debutanized distillate stream hasth following molar percentage composition:

to fractionating The fractionating The feed stock is supplied equipmentI0 maybe ofv any such as` a fractionating tower containing rubble Theequipment is operated'usually at about atmospheric. or somewhat aboveatmospheric pressure. In the diagram, equipment I0 is specifically shownas being a tower of multiplate construction containing bubble capplates. In normal operation of processingv according to the invention,about 50 platesare present in such a tower. The tower is shown as beingcomplete with an overhead vapor line I2, a condenser I3, a reflux drumI4, a reilux line I5, a bottoms line I6, a reboiler I8 and a reboilervapor line I9'. The

distillate product is withdrawn from the distillation system throughline 2| and the bottoms product from the system through line I1. Thesystem for the treatment of 25 lbs. per sq. in. (gauge), in order to beable to employ ordinary water for cooling purposes-in the condenser I3.The degree of separation in tower vII) is controlled by the heat supplyfrom the reboiler IB and the ratio of reflux supplied through line I5.The percentage of the feed taken ofi as product through line 2| isadjusted so as to take the major portion of the isoprene in thisfraction while rejecting most of th'piperylene and pentenes in theresidue. In the examplezused for illustration, a reflux ratio of 5 ismaintained while taking' oi approximately 19% of the feed as distillateproduct. Under these desired operating y the condensed distillate iswithdrawn from theconditions, the temperature at the top of the tower is141 F., while the temperature at the exit of the line I6 is about 210 F.

The overhead product removed through line I2 consists of a very narrowC5 fraction 'containing the majority of the isoprene and some of thepiperylene in the mixture, pentene-l and other Cs olefins and paraiiins.The entire distillate is condensed in equipment I3 and passed to thevdrum I4. From the drum I4, a quantity of reflux is passed through lineI5 in order to maintain the conditions. The remainder of drum Il andpassed through 1ine 2I for further processing. The ratio is between 5 to8, depending on the feed; In the processing of the feed stock of thespecific illustration, the-composition of the distillate material underthe specific operating conditions given shows the following molarpercentage composition: i C4 2.6 S-methyl butene-l 2.6 Pentene-l 49.92-methyl butene-l 10.4 Isoprene 19.9 Trans pentene-Z 2.1 n-pentane 5.8Cis pentene-Z 2.9 Z-methyl butene-2 2.9 Cyclopentadiene 0:6 Cis andtrans piperylene 013 Butenes 0.5 3-methy1 butene-l 0.5 Pentene-l 9.42-methyl butene-1 2.0 Isoprene 4.1 Trans pentene-2 1.0 N-pentane 018 Cispentene-'Z 1.0 2-methyl butene-2 2.2 Cyclopentadiene 1.3 Transpiperylene} 2 9 Cis piperylene Cyclopentene 2.1 Cyclopentane I0.1

C 11.7 Cs aromatics 11.7 C1 18.7 Cs 9.8 C9 20.2

of the usual types,

f ating tower 20.

The residual material consisting of most Iof the piperylene, someisoprene, normal pentane and some pentenes, in addition to higherboiling hydrocarbons, is withdrawn through line I5. A-portion of theresidue is passed through the reboiler I8 where it is vaporized in orderto supply the heat necessary to operate the tower, the vapor beingreturned to the tower I0 through line I9. The remainder of the higherboiling material is removed from the system through line I1.

The overhead fraction from tower I0 as removed through line 2| is passedto the fraction- The tower 20 is similar in design to tower III and maybe any type of suitable fractionating equipment. It is furnished with anoverhead vapor line 22, a condenser 23, a reflux drum V24, a reilux line25, a bottoms line 26, a reboiler 28 and a reboiler vapor line 29. Thetower also has a solvent line 39, a side stream vapor line 3| and a sidestream return line 36. The distillate product i's withdrawn from thesystem through line 4I and the bottoms product through line 21. Pressureupon the system is maintained at about 25 lbs. per sq. in. (gauge),inorder to have a workable condensing temperature similar trl)l thatprevailing in the fractionating equipment I The overhead distillatevapor removed through line 22 consists of azeotropic mixtures of C5olefins and paramns when aqueous acetone is employed as the solventmaterial admitted through line 39. The vapors are condensed in equipment23 and v passed to the reflux drum '24. A portion of the condensate isreturned through line 25 as reflux, while the remainder is withdrawnthrough line 4 I cooled in equipment 42 and conducted through line 43 tothe water scrubbing equipment 50.

The quantity of aqueous acetone admitted through line 39 to thedistillation system is for the. specific compositionof feed stock inv avolume ratio of solvent from the point of of the reilux to product tohydrocarbon mixture of 2-1 introduction to th'e plate upon 7 which theside stream is withdrawn. Below the plate from which the side streamproduct is withdrawn, the solvent is stripped of its hydrocarboncontent.

'I'he bottoms withdrawn through line 26 consists of relatively puresolvent. A portion of the bottoms liquid is passed through the reboiler28 wh'ere it is vaporized, the vapors being returned 'to the tower 20through the vapor line 29, to supply the heat necessary to operate thetower. The

remainder of the solvent, except for a small portion withdrawn forpurification, as to be further described, is conducted through line 21to cooler .38 and then through line 33 to the upper portion of the tower20. Thus, the major part of th'e solvent continuously recycles withinthe extractive distillation system.

'30v may be any suitable fractionating apparatus,

such as a tower provided with bubble plates, as previously employed. Thetower is provided 'with an overhead vapor line 32, a condenser 33, a

reux drum 34, a redux line 35 and a bottoms line 36, in addition to thevapor inlet line 3|. 'I'he vapor entering the tower through line 3|supplies the heat necessary to operate the tower.

'Ihe overhead distillate removed through line 32 consists essentially ofazeotropic mixtures of isoprene. acetone and water and contains inaddition small quantities of piperylene, cyclopentadiene and higherboiling materials such as polymers formed during the extractivedistillation operation. The vapor is condensed in equipment 33 andconducted to the drum 34. A portion of the condensate is returned to thetower 30 through line 35 as reflux; while the remainder is withdrawnthrough line 31 to the water scrubbingequipment 40.

l The degree of separation of the hydrocarbons taking place in theextractive distillationlsystem 20 is dependent on two functions, namely,the ratio of the solvent to hydrocarbon liquid on the plates and theratio of liquid to vapor flowing through the tower. The ratio 012solvent t0 hydro carbon is controlled by the rate of solventrecirculation. The ratio of liquid to vapor in the zone of high solventto hydrocarbon ratio, that is, between the solvent inlet and the sidestream outlet, is controlled by the reflux returned through line 25 fromthe reflux drum 24. Due to considerable difference in solventconcentration between the top of the tower and the extractivedistillation zone and the difference in latent heats of theconstituents, the reflux ratio and consequently the ratio of liquid tovapor at the top of the tower must be appreciably higher than the liquidto vapor ratio in the extractive distillation zone.

In the case of the specic example, the liquid to vapor ratio in theextractive distillation zone is 0.80, while the ratio at the top of thetower is 0.85, the latter being equivalent to a reflux ratio of 5.5. Thereflux ratio used in the side stream tower 30 is adjusted so as toobtain an overhead product consisting essentially of the azeotropicmixture of isoprene, acetone and water and to supply adequate reilux tothe solvent stripping section of the tower 20. Whichever requires thelarger amount of reux determines the reilux' ratio at the top ofthetower 30. In the case cited in the example, a reflux ratio of 20 ismaintained at the top of the tower 30, in order to supply adequate reuxto the bottoms solvent stripping section of the tower 20.

The heat necessary for the extractive distillation operation, as well asto concentrate the hydrocarbon fractions and to strip the solvent, issupplied completely from the single lreboiler 28. In this manner,considerable heat economy is ei'- fected, since the heat to operate theextractive distillation zone in tower 20 and the diolen concentratingside stream tower 30 are supplied by condensation og solvent atthe topof the solvent stripping section of the tower. If this were not done, acondenser would 'have to be providedfor the solvent stripper to supplythe reflux necessary for operation, and additional heat would berequired to operate the extractive distillation systern and to effectthe side stream concentration. In the same way, the condensation neededto produce the required reflux in the extractive distillation zone isobtained by supplying the heat needed to concentrate -the mono-olens atthe top of the tower 20.

In the operation of the system comprising towers 20 and 30 and theirauxiliaries, separation into the zones indicated above is clearlydefined by the various temperature zones. Thus, the vapor leaving thetop of the tower 20 is at 140 F. The temperature rises sharply to 155 F.between the top of the tower and the plate on which the solvent isadmitted through line 33, then very slowly reaching to 160 F. at theplate where the side stream is withdrawn through line 3|. Below theplate from which the side stream is removed and as a result ofthe'concentration of the `aqueous acetone, the temperature rises sharplyto 208 F. the temperature prevailing at the bottom,

of the tower. The temperature at the top of the side stream tower 30 is145 F. In tower 30, the same type of temperature gradient is shown as inthe concentrating section of the tower 20'.-

` The isoprene-acetone-water azeotropic mixture removed as distillatefrom the tower 30 through line 31 is conducted to the water scrubbingequipment 40. Equipment 40 may be any suitable type of a liquid-liquidcontacting device, in which countercurrent flow of the materials may bemaintained. In the present case, the tower is packed with Raschig ringsand sumcient pressure is maintained on the tower to prevent vaporizationof any of the materials passing through it. The stream removed from thesystem through line 31 is admitted to the bottom of the tower 40 througha distributing device and then allowed to now upward countercurrent to astream of water admitted to the tower through line 44.

In passing through the tower, the acetone is removed and the hydrocarbonis thus completely freed of the solvent employed in the extractivedistillation. The quantity of water employed is determined by thepartition of the solvent between f line 48 consists of a dilute solutionof acetone in water. This extract is combined with a similar extractfrom the tower 50 and treated for the -recovery of the acetone solventas to be subsequently described. l

The solvent-free d'iolefln concentrate from the tower 40 is conductedthrough line 46 to the fractionating tower 10. This tower may be anysuit- Pressure is maintained on the system so as to.

effect satisfactory condensation with ordinary water in the condenser13. vIn the specific ex-` ample, pressure maintained upon the tower isbetween 12 and 15 lbs. per sq. in. (gauge).

The distillate withdrawn through line 12 consists of highly purifiedisoprene containing traces of piperylene and pentenes. The distillate iscondensed in equipment 13 and the condensate conducted to the reflux'drum 14. A portion of condensate is returned to the tower 10 as refluxthrough line 15, while the remainder is withdrawn as product throughline 8l. It is cooled lin equipment I8 and transferred to storagethrough line l2.

'Ihe .bottoms consisting of the piperylene and cyclopentadiene, inaddition to any polymer or high boiling constituents of the feed to thetower and containing some isoprene, is withdrawn through line 16. Aportion of the material is passed to the reboiler 18 where itisfvaporized, the vapors being returned to the tower 10 through line 19yto supply the' heat necessary to operate the tower. The remainder ofvthe bottoms are removed through the line 11 and recycled to the feedtower I8 for the recovery of Ivany isoprene content.

By increasing the number of plates in tower 10 or increasing the refluxratio, all of the isoprene may be taken overhead as purified product. Ithas been found, however, more eflicient to reject a small amount ofisoprene to thebottoms and recycle same for recovery as described. Arefiux ratio of 12 is maintained for the desired separation inthe tower16.

The distillate fraction from the tower 20 which is withdrawn throughline 4l, containing the azeotropic mixtures of the C5 parafllns andolens is combined with any recycled solvent from.

the bottom of the tower 20 taken from line 21 which is Withdrawn throughline 43 to maintain the impurities in the solvent at a low value. In theexample, about 5% of the solvent passing through line 21 is withdrawnand combined with the distillate material removed through line 4i. Thecomposite solution is then cooled in equipment 42 and carried to thewater extraction tower 50 by means of the line 43. The extraction tower50 is similar to the extraction tower 40. It

is similarly packed with Raschig rings. The cornposite solution'consisting of the C5 olens and parains, in addition to acetone andwater, is introduced into the tower 50 near the bottom and passes upwardthrough the tower countercurrently to a stream of water introduced nearthe top through line 4'5.

In passing through the tower, the solvent is extracted from thehydrocarbon material. Any

polymers or other high honing material mine" of a dilute solution ofacetone in water is with.

drawn through line 48 at the bottom of the tower 50 and after beingcombined with a similar extract from the tower 40 is transferred throughline 5|, through heat exchangers 52 and line 6I to the solvent recoverytower 50.

Tower 60 -may be anyconvenient device for effecting fractionaldistillation, such as a tower containing about 20 bubble cap plates. Itis provided with an overhead vapor line 62, a condenser 63, a refluxdrum 64, reflux line 65, a bottoms withdrawal line 66 and a steam line68. The distillate product is withdrawn through line 68 and the bottomsthrough line 61.` The distillate from the tower 60 consists ofrelatively pure solvent. The tower is operated at about atmosphericpressure. The distillate vapor passes through line 62 to the condenser`63 and the condensate passed to reflux drum 64. A portion of thecondensate returned to the tower 6p through line 65, while the remainderisl withdrawn and transferred through line 69 to line 39 where itcombines with the solvent admitted to the tower 20. Heat is supplied tothe tower 60 by means of a steam line 68 through which open steam isinjected directly into the bottom of the tower. The bottoms comprisingthe major part of the water content of the feed and the condensed steamis withdrawn through line 66 to the heat exchanger 52. The heatexchanger 52 is also utilized to heat the feed to the tower 60. Thus,the ,sensible heat of the bottoms is utilized.

The reflux returned to the tower 60 through line 65 is controlled atsuch a rate that the water ratio of about 0.5 is maintained in the tower60.

With the foregoing disclosure and illustration of the invention it willbe obvious to those skilled in the art that various combinations andvariations of the applications of the principles described andillustrated can be made without departing from the inventive concept.

What is claimed is:

l. An improved process for the separation of isoprene of highpurity'from a hydrocarbon fraction rich in various Cs hydrocarbons, bothmonoolens and diolefins, yincluding isoprene, 2- methy1butene-2,cyclopentadiene, straight chain Cs dienes and cyclopentene, whichcomprises at least two distillation stages each conducted in a separatefractionating tower, one of said distillation stages being effected inthe absence of a volatility modifier wherein reflux ratio and number ofcontact stages are interadiusted to effect separation between adistillate fraction containing isoprene, Z-methyl-butene-l, pentene-l,Z- methylbutene-2 and substantially free of cyclopentene, and a secondfraction containing cyclopentadiene, straight chain Cs dienes andcyclopentene; andthe other distillation stage being conducted on saiddistillate fraction containing isoprene vand-in the presence of acetonein-v methylbutene-l, pentene-1 and 2-methy1-bu- `tene-2, separating theisoprene from the acetone and recycling' thevacetoned 4to'saiddistillation zone above the hydrocarbon feed supply.

2. An improved process for the separation of isoprene of high purityfrom a hydrocarbon fraction rich in variousgCs hydrocarbons, bothmonoolens and dioleflns, including isoprene, 2- methylbutene-Z,cyclopentadiene, straight chain C5 dienes and cyc'lopentene,V whichcomprises at least two distillation stages each conducted in a separatefractionating tower, one ofv said distillationV stages being eiected inthe absence of a volatility. modier wherein reflux ratio and number ofcontactv stagesfare interadjusted to effect separation between adistillate fraction tene-1, 2-methylbutene-2 and substantially free ofcyclopentene, and a secondfraction containing cyclopentadiene, straightchain Cs dienes and cyclopentene; and the other distillation stage beingconducted on said distillate fraction containi ing isoprene and in thepresence oi' a volatility modifying solvent selected from the class"consisting of acetone and furfural, introduced substantially aboveI thepoint of hydrocarbon feed supply in an amount suillcientto'maintain alarge proportion of at least two volumes of saidy solvent per rvolume ofhydrocarbon in the liquid phase present above the feed plate in thefdistillation zone in which the hydrocarbon fractiony anonce 12 ation isbeing effected,ywith hydrocarbon reux ratio, solvent to hydrocarbonratiok and ,number of contact stages interadjusted to veffect.substantially complete separation of isoprenefrom the '5 Cs olefins inthe said isoprene-containing fraction including Z-methylbutene-l,pentene-l and '2-'methy1butene-2, separating the isoprene vfrom Vthesolvent and recycling the solvent to said dism tillation zone above thehydrocarbon Ifeed sup- Ply; y

i JOHN a. PA'I'rERsoN.

HARCLD' W. SCHEELINE.

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